1. Field of the Invention
The present invention relates to an imaging lens which is to be installed in an image pick-up apparatus such as a digital still camera, a mobile phone with a camera, and a personal digital assistants (PDA) which utilizes an image sensing device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) and more particularly to a single focus imaging lens which can adjust focus.
2. Description of the Related Art
In recent years, miniaturization and increase in the number of pixels mounted have been in progress in image sensing devices such as CCDs and CMOSes. In conjunction with this trend, smaller and higher-performance image pick-up equipment and lenses installed thereon have been in demand. In the related art, many of imaging lenses to be installed on, for example, mobile phones with a camera are made up of three lenses. However, in order to cope with the increase in the number of pixels mounted on such image pick-up equipment, the number of lenses mounted in such imaging lenses tends to be increased. JP-A-2000-180719 discloses single focus imaging lenses which are made up of four lenses and five lenses, respectively. The imaging lenses are made up of, in order from an object side thereof, a frontlens group, an aperture stop and a rear lens group, and the rear lens group is made up of, in order from an object side thereof, a negative lens, a positive lens and a positive lens. In addition, the rear lens group has at least one aspheric surface.
In recent years, autofocus has been in demand even for imaging lenses such as those described above which are made up of a relatively small number of lenses for installation on mobile phones with a camera. In the related art, in these imaging lenses, since the number of lenses mounted is relatively small, not a zoom lens configuration but a single focus lens configuration has been adopted, and as the focus adjusting system, focusing by moving the whole lens system (along an optical axis) has generally been adopted. With focusing by moving the whole lens system, however, since the overall length is changed by focusing, there is caused a drawback that the construction of a lens barrel becomes complex and the lens system is enlarged. In the imaging lens described in JP-A-2000-180719, as is described in its 28th paragraph, the total focusing system is adopted. Note that although JP-A-2000-180719 states that only part of the imaging lens may be shifted for adjusting focus, there has been disclosed therein no specific embodiment. Since the imaging lens described in JP-A-2000-180719 is designed on the premise that the total focusing system is adopted, in the event that for example, an internal focusing system is attempted to be adopted in which part of internal lens components is shifted, the image plane changes largely as focus is adjusted, and hence, the imaging lens cannot cope with the increase in the number of pixels mounted.